A helicopter was also deployed, the department tweeted. Photos of the aftermath showed a collapsed roof and blackened and gutted interiors.
The cause of the fire was not immediately clear, and authorities are investigating.
Diego Seral, a spokesman for the national police, told reporters that the fire began at the Fonda Milagros nightclub — one of three adjoining clubs — and that it had sustained most of the damage, according to Reuters.
In a post on its Instagram page, the nightclub described the incident as “tragic” and said it was cooperating with authorities to determine what led to the blaze.
On Sunday, dazed relatives and friends waited outside the building for information on the missing.
One witness, who said his friends were still unaccounted for, told Reuters that he left moments before the fire alarm went off and the lights went out.
Three of the dead have been identified so far, Ballesta said at a news conference Sunday evening, and DNA tests are being conducted to identify the others.
The mayor declared three days of mourning for those who lost their lives and said King Felipe VI had conveyed his condolences.
Spanish Prime Minister Pedro Sánchez expressed “solidarity with the victims and relatives of the tragic fire” and offered assistance to local authorities.
In 1990, at least 43 people were killed when a blaze engulfed a basement disco in Zaragoza, in northeastern Spain.